The present invention relates to a curing method and apparatus for manufacturing beams of composite material with a J-shaped cross-section.
In order to construct complex structures in the aeronautical sector or for other applications it is often required to manufacture low-weight and high-rigidity beams made of composite material, with a J-shaped cross-section (FIG. 1). For some of these applications, for example for floor beams for passenger aircraft it is of particular importance to ensure the geometrical precision of the beam. Above all, it is necessary to ensure that the sill or bottom flange of the J-shaped cross-section is flat since the floor members and the core by means of which the beam is connected to the frames of the fuselage are attached thereto.
In order to manufacture the composite beams so-called curing tools are used, said tools consisting of supports able to give the beam its final form by means of the simultaneous application of pressure and heat inside an autoclave. In fact, the pressure allows compaction of the various layers forming the beam while the heat initially assists compaction, favouring fluidification of the resin, and subsequently activates the resin curing reaction which gives it its final structure. The curing tool has the function of supporting and containing the beam during these phases.
The current technology is based on the use of metal apparatus of the type comprising mould and counter-mould which completely surround the part (FIG. 2). The beam to be cured is positioned on one of the metal parts and enclosed by the other parts of the apparatus which can be disassembled and are movable. The whole assembly is then closed inside the so-called vacuum bag formed by a polyamide (nylon) film to which the vacuum is applied. The assembly is then placed inside an autoclave where a combination of pressure and heat with a predefined temporal progression is applied. The pressure applied to the vacuum bag is transmitted to the metal parts of the apparatus which in turn transmit the pressure onto the beam. As a result of this compaction, the pressure together with the simultaneous increase in temperature causes consolidation and cure of the resin.
The present state of the art has a certain number of drawbacks.                The application of pressure onto the beam occurs not directly but via rigid metal parts with the result that, if they are not perfectly joined together or their geometrical form does not correspond perfectly to the beam to be produced, greater pressure will be applied to the beam in certain zones and less pressure will be applied in other zones. This means that in higher-pressure zones the resin will be impoverished with a reduction in the local thickness of the beam, while in the zones where the pressure is lower there may exist poor compaction with possible porosity of the beam. This problem is particularly critical in the case of beams which have variable thicknesses to reduce weight.        An imperfect joint between the edge of the beam to be cured and the edge of the apparatus creates empty volumes into which the resin tends to flow as a result of the pressure, thus impoverishing the beam and causing a reduction in the thickness.        The apparatus has a considerable weight and therefore suitable handling means are required.        The apparatus composed of many components has high cleaning and maintenance costs.        